During the past twenty years, the developments in the field of manufacturing very large scale integrated circuits have been phenomenal. Circuits that used to occupy an entire room have been shrunken into a small integrated circuit, which may fit into, for example, a small calculator or computer. And still the aim of many in the industry is to further reduce the size of circuits so as to occupy even smaller areas.
As circuits are reduced in size, each component within each circuit is likewise reduced in size. The problem, however, lies in manufacturing these smaller components without degrading the function and performance of the circuits. This is where each component's line and space definitions are crucial in the production of these integrated circuits. Such line and spacing definitions are approaching dimensions near a tenth of a micron. Therefore, there is a need to have accurate equipment and manufacturing techniques in manufacturing these types of integrated circuits.
One equipment commonly used in manufacturing very large scale integrated circuit is a photostepper. A photostepper is used to expose a layer of resist disposed over a thin-film covered wafer to electromagnetic radiation spatially modulated with a circuit pattern. The photostepper usually steps and repeats the exposure of the resist so as to form images of multiple circuit patterns on the resist. The wafer is subsequently removed and the wafer is subjected to an etching process for removing the resist and part of the thin-film so as to leave a thin-film pattern disposed on the wafer defining the multiple circuit patterns.
The manner in which a photostepper exposes the layer of resist is by projecting an image of the circuit pattern towards the wafer. A reticle or mask having a series of darken images disposed thereon defining the circuit pattern is interposed between a light source and the wafer, and a controllable shutter is interposed between the mask and the wafer. The light source is constantly energized and the photostepper periodically opens its shutter so that electromagnetic energy having the proper wavelength emanating from the light source and propagating through the image on the mask strikes the resist. The shutter is thereafter moved to a different position over the wafer and another exposure is conducted. Generally, this is performed until an array of exposures is formed on the resist.
The exposing of the resist process is a critical step in obtaining well-defined circuit patterns on the wafer. This step becomes more critical as the dimensions of the lines and spacings of the circuit become smaller. In other words, precise exposure time and focus are required in order to optimize the image exposed on the wafer. A photostepper usually has multiple settings of exposure time and focus. One set of exposure time and focus settings being optimal in obtaining best circuit patterns. Therefore, there is a need to determine these precise settings of the photostepper to optimize the definitions of the circuit pattern disposed on the wafer.
The determination of the optimum focus and exposure settings is an on-going process. Usually, this determination should be conducted prior to manufacturing a production run of circuit patterns. The reason for constantly calibrating the photostepper is that various factors that affect the exposure of the resist change over time. For example, changes in barometric pressure or excessive opening of the photostepper's side panel may cause the optimum focus and exposure time settings to change. In addition variation in wafer, thin-film and/or resist also has an effect on the optimum settings of the photostepper. Therefore, there is a need for a technique that can be performed on a regular basis for calibrating the photostepper so that the optimum focus and exposure time settings are obtained prior to manufacturing a lot of production circuit wafers.
One technique used in obtaining these optimized settings is to form a focus/exposure matrix on the wafer. A focus/exposure matrix is a series of patterns disposed on the wafer usually arranged in rows and columns. A pattern may be formed by etching a thin-film disposed over the wafer, or by etching the top surface of a bare wafer, or preferably, by etching a layer of resist disposed over the wafer. Each pattern having been formed with a distinct focus and exposure time, as for example, the patterns in a row having been formed with substantially the same exposure time and a focus that is incremented between successive row patterns by an amount substantially corresponding to the focus resolution of the photostepper and the patterns in a column having substantially the same focus and an exposure time that is incremented between successive column patterns by a finite amount. By examining the focus/exposure matrix to determine the pattern that has the best defined lines and spacings, an optimal set of exposure time and focus settings may be obtained.
The disadvantage with this method is that the photostepper has a margin of error in its focus. The margin of error may be as high as plus or minus the focus resolution of the photostepper. As a result, there will be an uncertainty as to whether the increment in focus between successive patterns is uniform. For example, if one pattern has an error of plus the focus resolution, and the succeeding pattern has an error of minus the focus resolution, both patterns would be exposed with the same focus, although they were created using different focus settings. The margin of error problem is also applicable to the exposure time. Hence using this method is not reliable for photostepper calibration and checking. Therefore, there is a need to create a focus/exposure matrix that is more reliable.
A technique has been developed in the prior art to create a focus/exposure matrix that is more reliable. This technique uses the concept of overlapping exposures to improve the reliability of each exposed pattern. When exposures are overlapped, it produces an image that has an effective focus that is the average focus of the overlapping exposures. For example, if a first exposure has been created with a focus setting of zero micron and a second exposure has been created with a focus setting of 0.1 micron, then the effective focus of the overlapping of these exposures is the average focus, i.e. 0.05 micron. The margin of error is statistically improved by this averaging effect. For example, if the first exposure has a positive error and the second exposure has a negative error, these errors would cancel out at the overlapping region since the focuses are averaged. The only way that the overlapping region's effective error would equal that of each individual exposure is if both exposures have the extreme error in the same direction, and statistically, this is unlikely. The result is an improvement in the accuracy in the overlapping regions.
A prior art technique of forming a focus/exposure matrix using overlapping regions generally uses a substantially square frame to form each pattern. First, the photostepper is positioned over the wafer at a strategic spot, for example, over a spot near an upper-left corner of the wafer. The photostepper then exposes the resist with an initial focus and exposure time. This exposure may be designated exposure (1,1), for example, since it is the upper-left corner exposure of the focus/exposure matrix. Assume that exposure (1,1) was exposed with an initial focus setting of -0.1 micron. Then the photostepper is moved one-half a frame in the x-direction, and exposure (1,2) is exposed with a 0 micron focus setting. The photostepper again is moved one-half a frame in the x-direction and exposure (1,3) is exposed with a +0.1 micron focus setting. Because the photostepper moves one-half a frame for each exposure, the right-half of exposure (1,1) is overlapped with the left-half of exposure (1,2); and similarly, the right-half of exposure (1,2) is overlapped with the left-half of exposure (1,3). The resulting overlapping regions have an effective focus of -0.05 and +0.05 micron, respectively. The technique may be used to create a row of overlapping regions having an effective focus ranging from, for example, of -2.5 micron to +2.5 micron, in steps of 0.1 micron (i.e. the focus resolution of the photostepper). After a row of exposures is created, the photostepper moves one-half a frame in the y-direction and creates a similar row of exposures with a different exposure time. This process is repeated until a two-dimensional array of exposures is formed.
One disadvantage with this technique is that the effective focus of each overlapping region does not correspond to a focus setting of the photostepper. For instance, in the above example, the photostepper has a focus setting of -0.1, 0 and +0.1 micron. But the overlapping regions have an effective focus of -0.05 and +0.05 micron. The effective focus of each overlapping region always differs from the focus setting of the photostepper by 0.05 micron. It is difficult to determine the optimum focus setting from a focus/exposure matrix such as this one where none of the patterns have been formed with an effective focus corresponding to one of the focus settings. Therefore, there is a need for a technique to form a focus/exposure matrix having patterns formed with effective focuses corresponding to the photostepper's focus settings.
Another disadvantage with this technique is that the difference in effective focus between adjacent overlapping regions is only as good as the focus resolution of the photostepper. For instance, in the above example, the effective focus of the overlapping regions were -0.05 and +0.05 micron, respectively. The difference in effective focus for these regions is 0.1 micron (i.e. +0.05 micron-(-0.05 micron)). The focus resolution of the photostepper, which is defined as the difference in closest focus settings, is also 0.1 micron. To achieve better accuracy in the photostepper calibration process, it is desirable to reduce the difference in effective focus between adjacent overlapping regions.